1. Field of the Invention
The present invention generally relates to sources of coherent electromagnetic radiation and more particularly to improved solid-state devices capable of emitting coherent electromagnetic radiation at a frequency which is voltage controlled.
2. Description of the Related Art
In a recent publication entitled "Far Infrared Emission from Plasma Oscillations of Silicon Inversion Layers" by Tsui, D. C.; Gornik, E.; and Logan, R. A., published in the Solid State Communications of Volume 35, pp. 875-877; (Pergamon Press, Ltd.). The authors describe a source of coherent far infrared radiation. The source is provided by depositing a gold grating having a periodic spacing on the thin optically semitransparent titanium film gate electrode of a standard metal-oxide-semiconductor field-effect-transistor (MOSFET) having a silicon substrate. A gate voltage is applied to the gate electrode and the source-drain voltage is pulsed to excite the device into oscillation.
More particularly, the silicon substrate of a standard MOSFET is a p-type semiconductor material. The source and drain terminals of a standard device are usually respectively connected to two n-type regions formed by phosphor diffusion in the surface of the silicon substrate and spaced from one another. An insulating oxide layer of silicon oxide is formed on the substrate between the n-type regions, and a thin electrically conductive titanium layer is disposed on the oxide layer opposite the silicon substrate to form the gate electrode of the device. As is well known when a voltage is applied to the gate electrode thereby creating a potential between the gate electrode and the silicon substrate, electrons accumulate at the surface of the silicon substrate to form an inversion layer. Even though electrons are a minority in the bulk of the p-type silicon substrate, they become majority carriers on the surface, because of the high positive gate bias voltage potential applied across the insulating silicon oxide layer. Between this sheet of electrons under the gate and the p-type bulk, there is a gradual transition from the degenerate n-type surface characteristics of the inversion layer to the nondegenerate p-type (semiconducting) bulk. This gradual transition region is referred to as the depletion layer. In the depletion layer the electrons and holes are roughly in equal orders of magnitude of density and the material is highly resistant. The application of a sufficiently positive D.C. voltage to the gate electrode will increase the charge density of the inversion layer and thus make the inversion layer highly conductive. Under these circumstances a D.C. source-drain voltage would result in a high source drain current. A decrease of the gate voltage will conversely decrease the charge density of the inversion layer thereby decreasing the number of carrier electrons in the inversion layer. Thus, a smaller amount of drain current will flow at the same source-drain voltage.
For a sufficiently high positive D.C. gate voltage the inversion layer is in the order of 100 Angstroms thick. Since the surface area between the drain and source n-type regions are much larger than the thickness of this layer, the collection of electrons forming this layer can be thought of as a two-dimensional conductive layer with an adjustable current density.
It is known that the inversion layer, a two-dimensional plasma, can be excited into an oscillatory state by providing a pulsed signal between the source and drain terminals so as to create a two-dimensional plasmon. By providing a gold grating on the semitransparent gate electrode, an electric field couples the plasmon with electrons in the grating resulting in radiative decay of the two-dimensional plasmon and far infrared emission from the grating. The coherency of the emitted radiation is dependent upon achieving a constant charge density in the inversion layer, while the particular frequency of the coherent radiation emitted by the grating is a function of the total portion of the depletion layer in which a constant charge density is achieved. The total portion of the depletion layer in which a constant charge density is achieved is in turn dependent upon the gate voltage and the level of the source-drain current provided.
The reported observation of emission of coherent far infrared radiation from a grating-gated, large area, silicon-based MOSFET by Tsui et al is not suitable for commercial applications. Its main drawback is that the observations of coherent emission by the authors were made at liquid helium temperatures and not near or at room temperatures. The theory disclosed hereinafter, behind the silicon based MOSFET device, shows that for the long wavelength infrared region of the electromagnetic spectrum, the grating-gated, silicon substrated MOSFET has a maximum operational temperature of 150.degree. K well below room temperatures. Not having the theory explaining the phenomenon, Tsui et al could not develop a system which is operable at room temperatures which would also be self-exciting in response to predetermined levels of substantially constant D.C. voltage applied to the gate and source drain terminals.